Add rosdep documentation for expected package sources for OpenEmbedded

[robwoolley]

The documentation for rosdistro/rosdep has a section saying what sources are appropriate for each platform: https://github.com/ros/rosdistro/blob/master/CONTRIBUTING.md#expected-rosdep-sources

There was a suggest to add this for OpenEmbedded (https://github.com/ros/rosdistro/pull/40021#issuecomment-1969512630)

[jiaxshi]

That's a nice feature for OE users.